A major consideration in the design of microprocessor data processing systems is the system structure required for effective control of the various peripheral circuits being serviced and efficient data transfer to and from those peripheral circuits. In conventional data processing systems, the system structure associated with peripheral circuits usually has a high degree of specialized wiring and requires special purpose buffer circuits for interfacing between the peripheral circuits (which control peripheral devices controlled by the data processing system, such as displays, printers, keyboards, etc.) and the microprocessor data processing system. Various types of peripheral circuits usually have different requirements relative to the address selection of the peripheral circuit, the number of input pins, the number of output pins, and signal characteristics required for correct peripheral circuit operation. The use of special wiring and special purpose buffer circuits reduces the flexibility of the data processing system, since a change in the type of peripheral circuits interfacing with the data system may require that part of the system be rewired for the new circuit connection and that the section of the stored program associated with interfacing with the previous peripheral circuit be modified to suit the requirements of the new peripheral circuit. Recent development of compact and economical integrated circuit data systems involving a microprocessing unit (MPU) on a single chip has opened up broad areas of application where it was previously not feasible to use stored program digital data processing approaches. In many of the new application areas there are constraints which affect the design requirements of monolithic integrated circuits used to build MPU system. A MPU system may be required to interface with a wide variety of peripheral devices having many different electrical signal requirements for receiving and transmitting data. It is desirable that a standardized interconnecting scheme exist between an interface circuit and the MPU system such that wiring variations are not required for various system applications and a minimum number of wires and integrated circuit pins are used. MPU systems may be required to be capable of automatic operation such that an interruption in operation due to power supply failure or other system abnormalities can be overcome without manual intervention by an operator.